1. Field of the Invention
The present invention generally relates to a metal member coated with metal layers. More specifically, the invention relates to a metal member coated with metal layers to be used as an electronic member, an electric member, a mechanical member, a semiconductor member, a magnetic member, a member required to have excellent environment resistances, such as chemical resistance to acid and alkali, heat resistance and corrosion resistance, or a decorative member.
2. Description of the Prior Art
Conventionally, as a method for coating the surface of an underlying base metal member to improve various characteristics, such as electric contact resistance, solder wettability, wear resistance, mechanical strength, lubricity, environment resistance, chemical resistance, and decorative characteristics such as gloss, of the surface of the metal member, there has been widely carried out a plating process capable of obtaining high performance characteristics at relatively low costs.
In a typical plating process, a base metal material and a coating metal material are suitably selected to obtain desired characteristics. In particular, in order to combine materials having various excellent characteristics with respect to electric contact resistance, solder wettability, wear resistance, mechanical strength, lubricity, environment resistance, chemical resistance, and decorative characteristics such as gloss, at low costs, there has been carried out a method for electroplating a base metal material with an intermediate layer of a metal and further electroplating it with one selected from the group consisting of noble metals, such as Au, Ag, Pd and Rh, alloys containing them as principal components, Sn, Cu and Cr, and alloys containing them.
In particular, as a connector member having excellent wear resistance, mechanical strength, lubricity, environment resistance and so forth, there has been widely used a connector member wherein a base material is coated with relatively inexpensive Ni or an Ni alloy as a metal of an intermediate layer to be further coated with a noble metal.
As a connector member desired to have a high corrosion resistance and durability, there has been developed a connector member wherein an intermediate layer of Ni is coated with PdNi having an excellent corrosion resistance and wear resistance to be further plated with Au. In such a connector material having a high corrosion resistance and durability, the thickness of the Au layer is usually set in the range of 0.05 to 0.2 μm, which is required to decrease contact resistance at the minimum, in order to reduce costs by decreasing the amount of gold.
On the other hand, there is known a surface processing method for forming an electroless plating layer of an Ni—P alloy on the surface of a metal material, forming an electrolytic plating layer of an Ni—P alloy thereon, and forming a finishing plating layer thereon (see, e.g., Japanese Patent Laid-Open No. 3-229891). There is also known a lead frame which is produced by forming an Ni or Ni alloy plating layer on a lead frame base material, forming an Ni—P, Ni—B or Ni—Co alloy plating layer thereon, and forming an Au plating layer thereon (see, e.g., Japanese Patent Laid-Open No. 9-252070).
In recent years, it is desired to further reduce the cost of producing electronic materials. However, in the above described connector material wherein the Ni intermediate layer coated with PdNi is plated with Au, if the thickness of the Au plating layer is further decreased in order to reduce costs, there is a problem in that environment resistance is considerably deteriorated. In addition, if the Ni layer serving as the intermediate layer is simply thickened in order to reduce costs, the noble metal surface layer must be thickened since Ni is soft and has a low wear resistance and an insufficient corrosion resistance, so that costs are finally increased.
Therefore, the inventors have studied to replace PdNi, which is the second most expensive element after Au, with another inexpensive metal. However, if the NiP layer is used as the intermediate layer as described in Japanese Patent Laid-Open Nos. 3-229891 and 9-252070, although NiP is hard and has a higher wear resistance than that of Ni, there are disadvantages in that the deposition rate of NiP is far lower than that of Ni and that equipment costs are high when the thickness of the NiP layer is increased. In addition, if the NiP layer is used as a single underlayer, the corrosion resistance is insufficient, so that the noble metal surface layer must be similarly thickened to increase costs. Moreover, if the alloy plating described in Japanese Patent Laid-Open No. 9-252070 that the solder wettability can be improved is applied directly to a connector material to sequentially form an Ni alloy plating layer having a thickness of 1 μm, an Ni—P alloy plating layer having a thickness of 0.02 to 0.3 μm and an Au plating layer having a thickness of 0.1 μm, it is not possible to obtain a desired corrosion resistance although it is possible to improve wear resistance.